HUMAN RIBOSOMAL-RNA VARIANTS FROM A SINGLE INDIVIDUAL AND THEIR EXPRESSION IN DIFFERENT TISSUES

We have investigated the extent of sequence variation in human ribosom al RNA (rRNA) genes and the expression of specific rRNA gene variants in different tissues of an individual, Focusing on the fifth variable region (V5; nt 2065-2244) of the 28S rRNA gene, we find that sequence differences between rRNA genes of a single individual are characterize d by differences in number of repeats of simple sequences at four spec ific sites, These data support and extend previous findings which show similar V5 sequence variation in rRNA genes from a group of individua ls, We performed experiments to determine if there is differential gen e expression within the rRNA multigene family. From the analysis of da ta of six variant V5 probes protected from RNase digestion by rRNAs is olated from different tissues of the individual, we conclude that each variant rRNA is present in a similar proportion in these tissues, whe reas the actual contributions of variants differ, their relative propo rtion is maintained from tissue to tissue in an individual, We favor t he explanation of a gene dosage effect over that of a regulated gene e ffect to account for this pattern of rRNA gene expression, In addition , computer generated secondary structure models of each V5 clone struc ture predict the same three helix structure with the regions of sequen ce variation contained in one stem-loop structure.